1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electrophotographic copier, laser beam printer, or facsimile machine.
2. Description of the Related Art
There is a known electrophotographic image forming apparatus that electrostatically forms a toner image on the surface of a photo conductor serving as an image bearing member and electrostatically transfers the image to a recording material (for example, paper) in close contact therewith. In such an image forming apparatus, a conductive transfer roller or corona charger is used as a transfer member.
For example, the transfer member is pressed into contact with or brought in the proximity of a photo conductor, thus forming a transfer portion between the photo conductor and the transfer member. A recording material is passed through the transfer portion and a transfer bias voltage that has an opposite polarity to that of a toner image formed on the photo conductor is applied to the transfer member, thereby transferring the toner image on the photo conductor to the surface of the photo conductor.
Typical examples of the photo conductor for use in the image forming apparatus described above include an organic photo conductor (OPC) and an amorphous silicon photo conductor (hereinafter referred to as an “a-Si photo conductor”). The a-Si photo conductor is used as an electrophotographic photo conductor in, for example, a high-speed copier or laser beam printer because it has a high hardness, exhibits high sensitivity to a semiconductor laser, and also suffers very little degradation caused by repeated use.
The potential decay (dark decay) occurring after the completion of charging when the a-Si photo conductor is used is larger than that occurring when the organic photo conductor is used. It is well known that the potential decay characteristics of the photo conductor have temperature dependence. Therefore, it is well known that an exposure dose varied in consideration of change in potential decay characteristics dependent on the overall temperature of the image forming apparatus can be employed.
However, in known techniques, only overall temperature changes inside the image forming apparatus are considered. In operation of the image forming apparatus, temperature may be distributed inside the image forming apparatus by unbalanced arrangement of heat sources, such as a fixing device and motor, and airflow. As a result, uneven temperature distribution may be produced in the longitudinal direction of the photo conductor. In this case, because the degree of influence of temperature on the potential decay characteristics varies in the longitudinal direction of the photo conductor, even when the exposure dose is adjusted in accordance with the overall temperature of the image forming apparatus, as in known techniques, the photo conductor may not be optimally exposed in the longitudinal direction thereof. This problem is apt to be noticeable in an a-Si photo conductor, which is largely affected by temperature decay characteristics.